This invention relates to an acetabular cup of a total hip prosthesis which comprises a cup having a part-spherical inner bearing surface and a ball shaped head mounted on a femoral component with a diameter to cooperate with said inner bearing surface to allow free movement therein.
The ball is designed to accept an attachment fitting for location in a socket in the ball and such a fitting can, for example, be a femoral stem. Cups of this type are known which comprise a single element made, for example, of a synthetic plastics material or metal, the inner bearing surface can also be formed as in insert which again can be of a synthetic plastics material, metal, ceramic or metal ceramic composite material and which is carried in an outer housing, the housing engaging the acetabulum with which it is to be used and be held in place by, for example, cement or mechanical means, for example, nails or screws.
In other known constructions of this type of cup the inner bearing surface can be provided on an inner layer or insert made from a different material than an outer backing which engages the acetabulum. Dual mobility cups generally comprise an inner bearing surface which receives the part-spherical ball head and which is itself freely mounted in a part-spherical element which has outer and inner bearing surfaces, the outer bearing surface engaging the inner bearing surface of an outer housing which engages the acetabulum. This type of cup allows greater mobility of angular movement and anti-dislocation characteristics.
In all these constructions, and, more especially, bi-polar or dual mobility cups, it is advantageous to have means for retaining the ball head against the inner bearing surface. There are various ways of achieving this, for example with a hard material, there are various ways of retaining the ball head inside the inner bearing surface. Deformable material cannot be used and in one arrangement a ring or circlet is used but this requires a third part and with a risk of wear. Another arrangement can consist of having two flats on the head and turning the head at 90° prior to inserting it into the cup and then again turning the head back through 90°. A disadvantage with this type of construction is that specific heads are required and there is risk of wear due to the truncating of the head. A system of this type is shown in FR 2 785 525 and 2 807 315.
The present Applicant's U.S. Publication 2005/0228502, the disclosure of which is incorporated herein by reference, also shows a construction of this type in which the inner bearing surface of the cup extends over an angle of more than 180°. A portion thereof adjacent an entry mouth is formed with a substantially flat face which is at a radius from the center of the hemispherical inner bearing surface which is less than the radius of the remainder. The ball head has a co-operating substantially flat face on its part-spherical surface on which is provided means to receive and retain the stem with which it is to be used, and the prior attachment of the stem allows the ball head to be located in the cup and rotated but it is retained therein. The dimensions and configurations of the part-spherical inner bearing surface and the part-spherical bearing surface of the head being arranged to cause a movement of translation of the head during insertion when fitting.
This type of construction, although it provides a crescent shaped retention area at each opposed side of the head, relies upon the substantially flat face which is at a radius from the center of the hemispherical bearing surface to retain the ball in place.
The present invention is intended to provide an improved construction which provides better location of the ball.